Different dietary fiber (DF) sources elicit different physiological responses. The long term goal of this research program is to elucidate the mechanisms of action for dietary fiber. Giber's effects on the physiology and biochemistry in the gastrointestinal tract are a consequence of both its presence and its metabolism. However, net fermentation and the presence of undigested DF do not explain its effect on stool weight. DF is implicated in colon carcinogenesis, although its role is controversial in part because epidemiological and experimental data have been inconsistent. Absorption of fermentation products, short chain fatty acids (SCFA), has been demonstrated, although no DF is currently assigned a caloric value. The overall aim of this competing renewal is to determine the site, rate and extent of fermentation in monogastric species to test the hypotheses that is the rate, site and extent of fermentation and its effect on microbial growth and metabolism that determine which DF sources will increase stool weight, inhibit toxic or carcinogenic metabolic activity or provide net energy to the host. DF's rate of fermentation can be predicted in part by its solubility, although current analytic methods underestimate soluble DF reaching the colon. Some data indicate that DF affects secretion of small bowel mucin. Other data indicate that starch assimilation from legumes and processed grain products is incomplete. The relative proportion of soluble DF, mucin and starch reaching the colon will be determined as they would confound the relationships between DF fermentation and stool weight, microbial metabolism and net energy bioavailability. Experiments will be conducted in intact and colectomized rats, and in swine with cannulas chronically implanted in the gut. Plant-, bacterially- and endogenously-derived CHO will be distinguished on the basis of solubility, size and neutral and amino sugar composition. Relative rates and ratios of SCFA production will be determined to clarify proposed roles of SCFA in mucosal cell growth and in inhibiting hepatic cholesterol synthesis. Results from the proposed research will define optimal DF sources for the prevention and treatment of constipation and diverticulosis, and possibly for hypercholesterolemia and colon cancer. These experiments will provide evidence that some DF should be assigned caloric values and may need to be considered as energy sources in diabetic and weight reduction diets and products.